1. The Field of the Invention
The present invention relates to methods and systems for producing a gas-liquid mass transfer which, in one example, can be use for oxygenating biological cultures having a shallow depth within a reactor.
2. The Relevant Technology
The growth of biological cells within a bioreactor requires critical control over a number of different process parameters. For example, as cells grow, they absorb oxygen from the surrounding media and release CO2. The concentration of oxygen and CO2 within the media must be carefully monitored and regulated to ensure viability and optimal growth of the cells. Another factor that needs to be carefully monitored and controlled is the density of the cells within the culture. To make sure that all of the processing parameters are properly controlled, cells are typically grown in sequential stages of increasingly larger reactors. For example, cell cultures may initially start in a small flask. Once the cell density approaches a critical value, the culture is transferred to a larger bench top reactor where the culture is combined with additional media. In turn, once the cell density again reaches a critical value, the culture is again moved to a larger reactor with more media. This process continues until a desired volume of culture is achieved. Because each different sized reactor only processes the culture over a relatively narrow change in volume, conventional techniques can be used for controlling all of the process parameters.
Although the above method of production works, there are a number of disadvantages in having to transfer the cell culture to different containers during the growth process. For example, the process is time consuming, labor intensive, and requires that the producer obtain and maintain a relatively large number of different sized reactors. In addition, the process of transferring the culture temporarily halts the preferred processing conditions, can potentially damage the cells, and increases the risk of a breach in sterility. Attempts have been made to overcome some of the above problems by trying to process a large change in volume of culture within a single reactor. For example, in contrast to conventional reactors which may only see a change in the volume of culture by a factor of two, attempts have been made to increase the change in the volume of a culture within a reactor by a factor of five.
The concept is to start with a small volume of culture within a relatively large reactor container and then through batch or continuous feed mode continue to add media to the culture as the cells grow to a point where the container reaches a predefined maximum volume of culture. Depending upon how much culture is needed, the culture can still be transferred to a larger reactor. The goal is to reduce the number of different reactors/containers the culture needs to be transferred into before reaching the desired end volume.
There are, however, a number of complications in growing a culture within a single reactor over a large change in volume. For example, in each reactor there is a mechanism for oxygenating the culture, stripping out unwanted CO2, and continuously mixing the culture so that the culture remains substantially homogeneous. Mixing is commonly accomplished by an impeller disposed within the container. The impeller is sized, positioned and operated so as to achieve optimal mixing of the culture without damaging the cells. Oxygenation is typically accomplished by dispersing small diameter bubbles into the container holding the culture through a defined sparger located on the floor of the container. As the bubbles rise within the culture, the oxygen is absorbed into the culture. CO2 stripping is typically accomplished by dispersing large diameter bubbles into the container through a second sparger located on the floor of the container. As the large bubbles rise within the culture, a portion of the CO2 within the culture equilibrates into the air of the large bubbles and is carried out of the culture.
One of the complications of growing a culture within a single reactor over a large change in volume is that the parameters for oxygenating, stripping CO2 and mixing a culture, along with other operating parameters, change as the volume of culture increases. Traditional mechanisms, as discussed above, for oxygenating, stripping CO2 and mixing are designed to operate over a narrow range of fluid volumes and thus for a set configuration size do not effectively function at both small and large fluid volumes. The same is also true when other gases, such as nitrogen, are desired to be applied to the culture. Accordingly, what is needed in the art are methods and systems for oxygenating a culture and/or stripping CO2 from a culture and, more generically, creating a gas-liquid mass transfer with a culture that solves all or some of the above problems and can effectively operate in conditions where traditional sparger mechanisms have difficulty performing correctly.